1. Field of the Invention
The present invention relates to a gas sensor attached to a pipe which forms a flow path for a gas to be measured and adapted to detect a gas component of the gas to be measured by use of the air introduced therein as a reference gas.
2. Description of the Related Art
Conventionally known gas sensors for detecting the concentration of a specific gas component in a mixed gas include HC sensors and Nx sensors.
One type of an HC or NOx gas sensor is designated an outside air introduction sensor. This type of gas sensor includes a seal member located in its upper portion which includes an opening formed therein for introducing outside air into the gas sensor. An air permeable filter is disposed in the aperture and prevents entry of dust and water into the sensor. Since the filter is located away from the hot portion of the sensor, that is, that lower portion including a heater and that portion which is exposed to the measured gas, such as hot exhaust gas, thermal deterioration of the filter is prevented. The air permeable filter is sandwiched between an outer circumferential surface of a cylindrical insert and the inner surface of the aperture to partially close the aperture while maintaining air permeability but precluding entry of water. In such a gas sensor, in order to ensure air permeability which might be compromised by water, oil, dust or the like lodged in the opening, the filter generally projects a short distance from the atmosphere side face of the seal member. Thus, even if water, oil, dust or the like adheres to the filter, such substances will typically fall away and the air permeable portion of the gas sensor will maintain its impermeability to water and oil. The outward projection of the filter element prevents accumulation of dust on the filter and maintains good air permeability and performance of the sensor.
Nonetheless, such an outward projection of the air permeable filter from the adjacent seal member of the sensor may render it susceptible to damage when it is mounted in certain locations on the vehicle since it may be exposed to high pressure water utilized in washing the vehicle or may be exposed to a flipped stone which may break the filter resulting in serious damage to the gas sensor and compromised performance.
The present invention addresses and solves the above-mentioned problem. An object of the invention is to provide a gas sensor in which an air permeable filter is fixed to a seal member by use of a cylindrical insert member and which can prevent breakage of the air permeable filter and entry of foreign matter and can ensure air permeability of the air permeable filter.
To achieve the above object, the present invention provides a gas sensor comprising a seal member which, together with a casing, forms a reference gas space, and being configured such that a cylindrical insert member covered with a sheetlike air permeable filter is inserted into a through-hole formed in the seal member in such a manner as to extend between the atmosphere and the reference gas space. Thus, an atmosphere-side opening portion of the cylindrical insert member is closed with the air permeable filter, and the air permeable filter is fixedly sandwiched between the inner surface of the through-hole of the seal member and the outer surface of the cylindrical insert member. Passing through the air permeable filter, outside air is introduced into the reference gas space via an air path formed in the cylindrical insert member in such a manner as to extend between the atmosphere side opening portion and a reference gas side opening portion of the cylindrical insert member.
The atmosphere side opening portion of the cylindrical insert member is retracted from an end surface of the seal member, which is exposed to the atmosphere, toward the interior of the through-hole of the seal member, whereby the atmosphere side face of the air permeable filter is located within the interior of the seal member; i.e., is retracted from the end surface of the seal member.
Thus, the seal member intercepts high pressure water or a flipped stone directed toward the air permeable filter from the outside, thereby effectively preventing breakage of the air permeable filter. Therefore, the air permeable filter can favorably maintain impermeability to water, oil and the like and air permeability, thereby ensuring reliability of the gas sensor.
The seal member may be a cylindrical ceramic body whose inner and outer circumferential surfaces are covered with sheet rubber. Preferably, the entire seal member is formed of an elastic body such as a rubber member.
The thus formed seal member can absorb impact, to some extent, induced by impingement of high pressure water or a flipped stone, thereby alleviating impact on the air permeable filter fixedly disposed within the seal member and thus effectively preventing breakage of the air permeable filter.
The through-hole may extend through the seal member while its diameter is held constant. However, a through-hole of large diameter encounters difficulty in preventing entry of high pressure water or a flipped stone therein, whereas a through-hole of small diameter involves impaired air permeability. Also, since the cylindrical insert member is generally formed through deep drawing or like working, reduction of its diameter is limited for working related reasons, and thus reduction of the diameter of the through-hole is limited.
Thus, preferably, the through-hole of the seal member is constituted by a small hole opening to the atmosphere and a large hole opening to the reference gas space, the small hole and the large hole communicating with each other. The large hole has a diameter substantially equal to the outside diameter of the cylindrical insert member, and the small hole has a diameter smaller than that of the large hole.
The thus configured through-hole has an advantage that the small hole prevents entry of high pressure water or a flipped stone therein, while the large diameter can receive the cylindrical insert member. The cylindrical insert member abuts a step that is formed between the small hole and the large hole.
The diameters of the small and large holes are determined in view of, for example, suppression of entry of water and oil droplets and a required flow rate of air between the atmosphere and the reference gas space.
Meanwhile, the above mentioned configuration of the present invention requires the accommodation of the cylindrical insert member in the seal member. Generally, the seal member is elastically fixed between the casing and the cylindrical insert member through crimping of the casing from the outside. Therefore, preferably, the cylindrical insert member is present at least at a position corresponding to a portion of the seal member subjected to crimping. Also, in order to impose a certain crimping force or greater on the seal member, the seal member must be subjected to crimping along a predetermined length or longer.
Thus, in order to implement the above mentioned configuration of the present invention, the axial length of the seal member must be rendered longer than that of the cylindrical insert member. However, increasing the length of the casing accordingly is contrary to demand for compactness of a gas sensor. In this case, axially elongating only the seal member is possible. However, thermal expansion of the elongated seal member in the course of using the gas sensor at high temperature induces local stress at a portion of the seal member in contact with an end portion of the casing, potentially resulting in breakage of the seal member at the contact portion.
Therefore, preferably, the seal member has a protrusion axially protruding from the atmosphere side end face thereof, and an atmosphere-side opening portion of the cylindrical insert member is inserted into the protrusion.
Employment of the protrusion allows the seal member to be fixedly disposed between the casing and the cylindrical insert member in a stable condition while the cylindrical insert member is accommodated in the interior of the seal member. Thus, the above-mentioned configuration of the present invention can be effectively implemented.
According to the above-mentioned configuration of the present invention, the cylindrical insert member is fixedly disposed in the interior of the seal member to thereby dispose the air permeable filter within the through-hole of the seal member. Thus, depending on the size of the through-hole or the installation environment of the gas sensor, water, oil or the like may accumulate in a recess defined by the atmosphere side face of the air permeable filter and the wall of the through-hole, resulting in a failure to maintain air permeability.
Therefore, preferably, one or more drain grooves are formed on the atmosphere side end face of the seal member in such a manner as to extend radially outward from the periphery of the atmosphere side face of the air permeable filter, so as to drain radially outward water or oil adhering to the atmosphere side face.
Employment of the drain groove prevents a problem in that adhering water or oil stagnates on the atmosphere side face of the air permeable filter, thereby ensuring air permeability and thus maintaining good gas sensor performance.
Preferably, the bottom surface of the drain groove is located in such a manner as to be biased toward the reference gas space in relation to the atmosphere side face of the air permeable filter. This feature of the drain groove enhances radially outward drainage of water or oil via the drain groove particularly when the gas sensor is installed such that a proximal end portion thereof faces upward.